According to the Soil Association, the resistance to the colistin antibiotic is considered to be a ‘major step towards completely untreatable infections’.
Public Health England found resistant bacteria in samples of human infections and on three farms. Officials said the threat to public health was low, but that they would be monitoring the situation closely.
The colistin resistance gene, called mcr-1, was first found last month in China in pigs, retail meat and human infections.
The agricultural sector has for some time been blamed for its part in antibiotic resistance in both humans and animals.
Colistin is frequently used for mass medication of intensively farmed pigs and poultry, and scientists believe the resistance gene has spread from farm animals to humans because the antibiotic is used much more widely in veterinary medicine than it is in human medicine.
A Freedom of Information request submitted by the Alliance to Save Our Antibiotics found 837kg of colistin were sold for use in British farm animals in 2014, whereas just 300kg are used per year in human medicine.
In Europe as a whole, the amount used in farm animals (545 tonnes) is more than 500 times higher than the amount used in humans (about 1 tonne), with use in farm animals in Spain (177 tonnes), Italy (133 tonnes), Germany (124 tonnes) and France (50 tonnes) being particularly high.
Cóilín Nunan, scientific adviser to the Alliance, said: “Despite scientists saying that resistance to this last-resort antibiotic is likely to be spreading from farm animals to humans, it still remains completely legal in the UK and in most EU countries to routinely feed colistin to large groups of intensively farmed animals, even when no disease has been diagnosed in any of the animals.
“We need the government, the European Commission and regulatory bodies like the Veterinary Medicines Directorate to respond urgently.
"The routine preventative use in farming of colistin, and all antibiotics important in human medicine, needs to be banned immediately.”